Broadband light sources in the mid-infrared (MIR) region (2 um to 10 um) are used, for example, in remote sensing, IR-counter measures, medical diagnostic and spectroscopy applications. While incoherent broadband MIR sources have been available for years and are used in spectroscopy, these sources have limited power spectral density and poor beam quality. Coherent broadband light sources based on nonlinear spectral broadening, widely known as super-continuum sources, have been studied in the visible and near-infrared (NIR) region of the optical spectrum. Recently, there has been interest in developing such super-continuum sources in the MIR region. Such super-continuum sources should have high power density, high beam quality and low noise (high coherence) in order to offer significant advantage over existing incoherent MIR sources.
Super-continuum sources in the MIR region have been realized and reported in current publications. Such systems can be placed in two general categories primarily in connection with the type of pump source that they use: (a) systems using a nanosecond or picosecond pulsed laser as the pump source, and (b) systems using a femtosecond pulsed laser as the pump source. It is generally understood that the systems falling into category (a) suffer from low shot-to-shot coherence. This lack of coherence is particularly evident in the noise characteristics of the generated spectrum and leads to spectral and temporal fluctuations from shot to shot. While these low-coherence systems are useful as powerful broadband light sources with high beam quality, their application in spectroscopy is largely limited due to the coherence problem. The systems falling into category (b) can be designed to have high coherence by carefully adjusting the properties of the femtosecond pump pulse as well as the nonlinear medium used for broadening the spectrum. Concerning the pump sources for this category, there are a number of laser systems used in prior art. It is desirable to have the pump wavelength in close proximity or within the wavelength region where the super-continuum is generated. Two types of femtosecond sources that have been used for MIR super-continuum generation include mode-locked fiber lasers based on Thulium or Holmium doped fibers, and optical parametric oscillators. The fiber lasers provide femtosecond pulses with high energies at a center wavelength close to 2000 nm and potentially out to 3500 nm. The fiber lasers used for this application in prior art had a fixed wavelength. In addition, the mode-locking mechanisms for these fiber lasers are still under research and development and the number of commercially available devices is limited. The optical parametric oscillators (OPO) provide femtosecond pulses with a tunable center wavelength. However, the OPOs are expensive systems that occupy a large space. Additionally, the average powers available from OPOs are limited when compared with fiber-based sources.
Therefore, there is a need for a low-cost and compact system to generate femtosecond pulses for mid-infrared super-continuum generation. Additionally, a method for adjusting the pulse parameters such as wavelength, peak power, energy, and polarization is required in order to optimize the spectral brightness, bandwidth, spectral flatness, and coherence of the super-continuum.
One concrete spectroscopy application would be to use the broadband sources in conjunction with a Fourier transform spectrometer and a sample processing unit. There has been recent development and commercialization of Fourier transform spectrometers in the MIR region. By developing the low-noise MIR broadband source, complete spectroscopy systems for the MIR can be provided, which would offer a significant advantage over existing spectroscopy systems.